The present invention relates to a magnetic disk drive having a composite magnetic head made up of a write head and a read head, and more particularly to a mechanism for, in a write operation, detecting a head offset even in a data region between servo regions.
The following procedure is used to write data in a magnetic disk drive employing a “data surface servo system” in which data recording regions and servo information regions are formed on the same magnetic disk medium. First, the magnetic disk drive positions the write head at the track that includes the sector to which to write the data. Then, the drive reads the servo information until the target sector is reached, thereby obtaining the disk radial position of the head. The drive then calculates the amount of displacement between the ideal radial position and the current radial position (hereinafter referred to as the offset amount) and if it exceeds a predetermined threshold value, the drive does not write the data to the sector. If, on the other hand, the offset amount is within the threshold value, the drive writes the data to the sector. Furthermore, after writing the data, the drive obtains an offset amount from the next servo information and determines whether it is within a predetermined threshold value, as in the above. If the offset amount is within the threshold value, the magnetic disk drive determines that it has been able to write the data to the proper position. If, on the other hand, the offset amount exceeds the threshold value, the magnetic disk drive determines that it has not been able to write the data to the proper position and hence performs processing for rewriting the data.
Thus, the conventional magnetic disk drive writes data to the disk without detecting the head offset amount in the data recording regions between servo information regions. This means that even when a head offset has occurred within a data recording region, the magnetic disk drive writes data without correcting the head position until it reaches the next servo information. In such a case, the data is not written to the proper position on the target tack, and data on an adjacent track(s) may be lost. If this happens, the drive cannot properly read data from the adjacent track(s). Further, assume that a head offset occurred in a data recording region but the head has returned to the proper position when it has reached the next servo information region. In such a case, the drive erroneously determines that the data has been properly written. This leads to an inability to read the proper data in a data read operation.
Various techniques have been used to prevent data from being written when a head offset has occurred in a data recording region. For example, when servo information obtained right after a data write operation to a data recording region indicates that a head offset has occurred between the current and previous servo information regions, the technique disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2001-14606) determines the offset direction based on the servo information and performs processing to prevent loss of data from the track on the offset side of the current track. Further, the technique disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 05-133974 (1993)) detects external shock by using an acceleration sensor, and upon detecting a shock, this technique stops the current data write operation, since a head offset may have occurred in a data recording region (between servo regions), thereby increasing the data writing reliability.